Tests: Improve full-stack testing without mocking whole modules

[amotl]

Previously, the test suite used the surrogate package, because it was not ensured that all third-party packages would have been installed within the testing environment.

Now, with b4c461977, all third-party packages will always be installed when installing the "test" extra, in order to enable full-stack testing, and improve the overall quality of the test suite.

702fa768d9d disables testing the apns service plugin, because the apns package is not ready for Python3. For upgrading to a successor library, see also #580.

[codecov-commenter]

Codecov Report

Merging #615 (b839aa7) into main (aa9f8dd) will decrease coverage by 0.88%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #615      +/-   ##
==========================================
- Coverage   37.56%   36.68%   -0.89%     
==========================================
  Files          80       80              
  Lines        3604     3604              
==========================================
- Hits         1354     1322      -32     
- Misses       2250     2282      +32     

Flag	Coverage Δ
unittests	36.68% <ø> (-0.89%)	:arrow_down:

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Impacted Files	Coverage Δ
mqttwarn/services/apns.py	0.00% <0.00%> (-100.00%)	:arrow_down:

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